1. Field of the Invention
The present invention relates to a control apparatus, a control method, and a computer program. More particularly, the present invention relates to a control apparatus, a control method, and a computer program for easily reducing an unnecessary portion of auto-focus process.
2. Description of the Related Art
Known image pickup apparatuses are typically provided with an auto-focus (AF) function to automatically focus on a subject. International Patent Publication No. WO 97/25812 discloses a technique of AF process that detects a contrast of a captured image of a subject, and adjusts the position of a lens to reach the highest contrast (image processing technique). A captured image is in focus on the subject within a field of view.
The AF process is started when a user presses a shutter button half the stroke thereof, and is completed when the captured image is determined to be in focus on the subject. Such an AF process that is performed each time the user issues a command is referred to as a one-shot AF process.
An image pickup apparatus, such as a video camera, performs the AF process without any command from the user. Such an image pickup apparatus repeatedly performs condition determination operations in the AF process, and starts the above-referenced AF process when a predetermined condition is satisfied. One of startup conditions is a change in an assessment value of contrast. For example, an image pickup apparatus assesses contrast in a predetermined area (assessment frame) within the captured image, and starts the one-shot AF process when a change in the assessment value becomes equal to or above a threshold value. The image pickup apparatus can thus continuously be in focus on a subject within the assessment frame by assessing the contrast of the captured image and controlling the AF process based on the assessment value.
A variety of methods of performing appropriately the AF process is contemplated. For example, Japanese Unexamined Patent Application Publication No. 10-215403 discloses one technique in which a lens is wobbled within a tiny range to efficiently search for an optimum focus position and, and the direction of movement of the lens is determined based on an assessment value of the captured image. Japanese Unexamined Patent Application Publication No. 10-161016 discloses another technique. According to the disclosure, the size of an assessment frame over which an assessment value is calculated is modified based on conditions so that an optimum AF process is performed within a wide range of photographing conditions.
In such image processing type AF processes, the image pickup apparatus moves the lens to be in focus on the subject while viewing the captured image. A length of time is thus required before the AF process is completed (with in-focus state reached). For example, when the image pickup apparatus changes the direction of photographing (during panning or tilting), the subject within the assessment frame successively changes. From when the image pickup apparatus determines that the in-focus state to the subject has been reached in the AF process to when the AF process is actually completed by moving a lens to the in-focus state, distance to the subject photographed in the assessment frame can vary, leading to an unsuccessful focusing operation.
As shown in FIG. 1A, a camera 1 now pans, successively photographing a subject A, a subject B, and a subject C as represented by an arrow-headed curve line 2. Each of the distance 3 from the camera 1 to the subject A and the distance 5 from the camera 1 to the subject C is shorter than the distance 4 from the camera 1 to the subject B. The assessment value of the image captured by the camera 1 during panning changes a plot 6 of FIG. 6B.
The distance 4 is longer than the distance 3. The assessment value substantially drops at time t2 the subject B is photographed within the assessment frame, and the AF process is thus started. The panning continues, and the subject C is photographed in the assessment frame when the lens driving is almost ended. More specifically, the camera 1 fails to be in focus on the subject C with the focus position remaining in alignment with the distance 4. At time t3 thereafter, the assessment value becomes stabilized, keeping the AF process untriggered. The image is thus captured with the subject C out of focus. The same is true of a tilting operation.
Out-of-focus problems also take place in cases other than the panning and tilting operations. As shown in FIG. 2A, a subject 12 is photographed in an assessment frame 11 in a photographing frame 10. The subject 12 now repeatedly enters and the leaves the assessment frame 11 as a result of wind, thereby swinging laterally rather than fore and aft. The assessment value thus greatly varies as represented by a plot of FIG. 2B. The AF process is frequently started. In the AF process, lens is driven, causing the focus position to be moved. As a result, the captured image is hard to view with the focus position remaining unstable.